1. Field of the Invention
The present invention relates to a focus adjuster which prevents machining defects caused by changes in focal position of a machining lens due to contamination of the machining lens in order to make it possible to use unskilled operators in operating a laser beam machine, maximize quality and productivity for each material and workpiece thickness, and accomplish long, unattended operation under optimum machining conditions.
2. Description of the Related Art
Conventionally, focal position of a laser beam narrowed by a machining lens is set beforehand in the thickness direction of a workpiece (e.g., along a Z axis). To set the focal position of the machining lens, a laser beam of appropriate power is emitted continuously and the machining lens is brought close to the workpiece surface, and when focus is achieved, plasma rays (blue rays) are generated on the workpiece surface. When the operator recognizes the plasma rays, he/she stops the movement of the machining lens in the thickness direction and inputs this position as a standard focal position in an NC system (number control system).
There is a known technique which detects the plasma rays generated during a focusing operation, using an optical sensor installed in a nozzle mount of a machining head, and thereby adjusts the focal position automatically (see Japanese Patent Laid-Open Publication No. 6-7980 (Patent Document 1).
The focal position of the machining lens set in this way is not always constant, but varies due to contamination or the like of the machining lens, causing a deviation between standard focal position and optimum focal position and resulting in machining defects such as degradation of machining quality. Consequently, the operator must manually adjust the focal position of the machining lens in the thickness direction based on experience.
Furthermore, to adjust the focal position of the machining lens, a torch must be removed manually.
Consequently, the machining lens gets contaminated, being open to the air in the factory.